Conventionally, a gas sensor is disposed in an exhaust passage (exhaust pipe) of an internal combustion engine of an automobile or the like, and the air-fuel ratio is controlled by detecting the concentration of a specific gas contained in exhaust gas through use of the gas sensor. Specific examples of such gas sensor are an oxygen sensor for detecting the oxygen concentration of exhaust gas and an NOx sensor for detecting the concentration of nitrogen oxide (NOx). Such an oxygen sensor includes a detection element having at least one cell composed of an oxygen-ion conductive solid electrolyte member and a pair of electrodes formed thereon. Since the cell formed of a solid electrolyte member exhibits good oxygen ion conductivity when it is heated to a predetermined temperature (activation temperature), a heater for heating the detection element is provided. In order to maintain the active state of the oxygen sensor (cell), the temperature of the detection element is fed back to a controller so as to control the electric power supplied to the heater for heating the detection element (element temperature feedback control). This element temperature feedback control is performed on the basis of the impedance of the detection element which changes with the temperature thereof. At that time, an impedance corresponding to the activation temperature is used as a target impedance. However, when the oxygen sensor deteriorates, the impedance of the detection element increases. In such a case, even when the temperature of the oxygen sensor is the activation temperature, due to the increased impedance of the detection element, the electric power supplied to the heater is increased such that the detection element has the target impedance. As a result, the temperature of the oxygen sensor increases. This temperature increase raises a problem of accelerating the deterioration of the oxygen sensor.
In order to solve such a problem, there has been proposed an oxygen concentration detection apparatus which includes heater-supplied-power control means for feedback-controlling the electric power supplied to the heater such that the impedance of the detection element, which changes with the temperature thereof, becomes a target impedance; deterioration determination means for determining whether or not the oxygen sensor has deteriorated on the bases of an increase in the impedance of the detection element; and target impedance change means for changing the target impedance to increase when the oxygen sensor is determined to have deteriorated (see, for example, Patent Document 1). When the oxygen sensor has deteriorated, the electric power supplied to the heater is controlled by changing the target impedance to a new target impedance, to thereby prevent the temperature of the oxygen sensor from increasing. Thus, the oxygen sensor can be maintained at a target element temperature.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No H10-26599